The invention described herein relates to an adhesive paste for bonding higher power devices to a substrate with a minimum of thermal resistance through the adhesive bondline and the method of use.
The attachment of active devices, such as semiconductor dies, to a substrate or in an electronic package has historically been accomplished using organic or inorganic adhesives. Typically, a semiconductor die is bonded to a substrate that is part of the semiconductor package. The adhesives that accomplish this bonding or attachment are usually in the form of a wet paste which consists of (1) a binder or adhesive component, (2) a filler, and (3) an optional diluent.
The purpose of the adhesive is to create a bond between the semiconductor device and the substrate. This adhesive layer between the device and substrate is often referred to as the bondline. Inorganic and organic binders have both previously been used. The most common inorganic binders are glass, solder, or a eutectic of Si and Au. Generally, this type of binder is used in inorganic packages, such as ceramic or metal. They are not generally adaptable to bonding die in plastic packages because of the high temperature processing required. The lowest attach temperature, for example, with Ag filled glass is about 300.degree. C. as described in U.S. Pat. No. 4,933,030. This processing temperature, however, is too high for use with a plastic laminate package. On the other hand, the thermal conductivity of these inorganic adhesive systems is high, ranging about from 40 to 65 W/mK.
Organic binders are the most common type of binders used for die attach in plastic packages because of their low processing temperatures. However, the thermal conductivities of prior organic adhesive systems have been limited to about 3-4 W/mK, thus limiting the amount of heat dissipation possible in plastic packages.
Organic binders can be further classified into thermoset and thermoplastic. Thermoplastic organic binders are described in detail in U.S. Pat. No. 5,391,604. Thermosetting binders, as the name implies, "set up" or "crosslink" during a thermal excursion. Typically, a thermoset adhesive contains an epoxy resin, a hardener, a filler, and in many cases a small amount of reactive solvent. The function of the hardener is to crosslink with the epoxy resin and change from a liquid to a solid. More recently, as described in U.S. Pat. No. 5,480,957, the hardeners can be "latent" i.e., they do not react with the resins until well above room temperature. The use of a latent hardener extends the pot life or time at room temperature before the viscosity increases to an unacceptable level for dispensing as a result of room temperature crosslinking.
Other ingredients sometimes used in thermoset adhesives in combination with an epoxy resin include a reactive diluent or solvent. The reactive solvent is typically an unsaturated low viscosity monomer capable of crosslinking with the epoxy and becoming part of the hardened structure. It is used to lower the viscosity of the epoxy resin to allow more loading of the filler. Thus, in the case of electro conductive adhesives, in which the filler usually is silver, higher electrical and thermal conductivities are realized. However, because of the lower molecular weight of the reactive solvent, only a limited amount can be used without sacrificing heat resistance and other functional properties. Thus, thermal conductivities have been limited to about 3-4 W/mK in prior art organic adhesives.
In some cases, a fugitive non-reactive solvent is used with an epoxy resin. For example, U.S. Pat. No. 3,716,489 describes a volatile solvent used to dissolve a photochromic material in an epoxy resin to form a transparent epoxy photochromic filter. Unlike the invention described herein, the above material is not used as an adhesive nor is it loaded with a particulate conductive filler.
U.S. Pat. Nos. 5,011,627 and 4,564,563 further describe screen printable electro conductive pastes that include fugitive, non-reactive solvents that are used to dissolve the thermoplastic and epoxy resins. These examples, however, are used as conductive traces, such as on membrane keyboards, rather than adhesives and hence the solvents are easily evaporated before entrapment in the structure by the crosslinking. Thus, they could not be employed as adhesives wherein the solvents must be volatized from the bondline between the die and substrate before crosslinking occurs to avoid entrapment of the solvent and voids.